In recent years, the elucidation of disease causes is proceeding on a gene level, and the advances in medical treatment have made brilliant achievements. Keeping in step with these changes, the developments of prescription products vary widely so that a need has arisen to inject an extremely small quantity of a drug solution into the body by a syringe. In such a situation, a micro-volume syringe that the quantity of a drug solution to be filled in a syringe barrel is, for example, as little as 1 mL or smaller or even 0.25 mL is used. As in the before, such a micro-volume syringe is also comprised basically of a syringe barrel, a piston slidable in the syringe barrel, and a plunger rod for pushing the piston.
In general, a piston and a plunger rod, which serves to cause the piston to slide, are connected to each other by keeping a threaded portion, which is arranged at a tip of the plunger rod, in engagement with a threaded portion in a cavity which the piston is provided with (see, for example, Patent Document 1). In view of its clinical application, the piston is required to have performance to be described hereinafter. Firstly, contamination of a drug solution from the material forming the piston needs to be avoided, and therefore, the material forming especially a forward end portion that comes into contact with the drug solution is required to have excellent chemical resistance. Further, the piston connected to the tip of the plunger rod is required to be smoothly slidable on and along a wall of a syringe barrel without causing leakage upon filling the drug solution into the syringe barrel or ejecting it from the syringe barrel. As the performance of the piston, smooth slidability is hence required in addition to sealing performance.
Concerning the piston-forming material required to have the above-described performance, it has been proposed to cover a surface of a piston, which is formed of a synthetic rubber, with a fluorinated resin (see Patent Document 2). However, a plunger rod arranged in threaded engagement with a piston may accidentally slip off from the piston when the plunger rod is handled to pull out the piston from the syringe barrel. This problem pronouncedly arises when a piston formed of such a covered synthetic rubber is used. Proposals have also been made to avoid such a problem (see, for example, Patent Document 3). It is to be noted that the manner of connection between the piston and plunger rod is not limited to their threaded engagement but includes fitting an anchor-shaped portion, which is arranged on the tip of the plunger rod, in a cavity arranged in the piston (see, for example, Patent Document 4).